Everybody forgets that the STEN was a War Time Emergency, "We need an SMG, right away quick!", thing that cost next to nothing to produce(~ $10 each. VS $225 for a Thompson. $24 for an MP40.). It was known as the Plumber's Nightmare for a reason.
"...feed problems..." With a 75 plus year old mag that's been knocked around by All the King's Men and milsurp dealers everywhere since it was new? Brand spanking new mags can have feeding issues too. Usually fixed with needle nosed pliers.